The growth hormone (GH)gene is specifically expressed in the somatotrophs of the anterior pituitary. This highly restricted expression is largely due to the action of a cell-type specific promoter, recognized by the pituitary specific transcription factor GHF-1. During the previous grant period GHF-1 was purified to homogeneity and its complete amino acid sequence was deduced after isolation of corresponding cDNA clones. A variety of experiments carried out both in vivo and in vitro indicate that the presence of GHF-1 is positively correlated with expression of GH. Furthermore, GHF-1 appears to be responsible for activation of the GH gene during fetal development. So far, it seems that the cell-type specific expression of the GH gene is controlled by the presence or absence of GHF-1. Therefore, the key to complete understanding of the cell-type specific activation of the GH gene and differentiation of somatotrophs lies within the mechanisms that regulate GHF-1 expression. To understand how GHF-1 expression is regulated we will study the mechanisms responsible for activating transcription of the GHF-1 gene in a temporal and spatial specific manner. The cis acting elements and the proteins which recognize them will be identified and characterized in detail. Genes encoding trans-acting factors controlling GHF-1 expression will be cloned and their expression pattern during mouse development will be analyzed. We will also investigate modulation of GHF-1 expression by various hormone signals. GHF-1 expression is also controlled post- transcriptionally probably at the level of translation. The mechanism and factors involved in controlling GHF-1 translation will be studied using in vivo and in vitro approaches. In addition to studying how GHF-1 expression is controlled it is also important to understand how the binding of this factor to the GH promoter results in activation of GH transcription. In this respect we will determine whether ectopic production of GHF-1 is sufficient for activating transcription of GH in cells which normally do not express this gene. We will also examine the effect of GHF-1 on the - chromatin structure of the GH and identify other proteins with which GHF-l interacts to activate GH transcription. We strongly believe that completion of studies will provide us with detailed molecular knowledge of the mechanisms responsible for the highly specific manner of GH transcription and its modulation in response to various hormonal signals and trophic factors.